Fin fish are commonly raised in an aquaculture setting, which may include a cage floating in a river or open ocean, or a water-filled tank. There is a need for the fish farmer to estimate the size of the fish being raised, to help determine whether they are growing properly, are ready for harvest, etc. It is usually desirable to provide an objective determination of fish size, to avoid variations caused by different persons making the estimate or a person being influenced by extraneous factors. A simple system that objectively indicates the size of fish within a limited environment such as within a cage, would be of value.